a) Field of the Invention
The present invention relates to a cell gasket, and more particularly to a cell gasket capable of preserving a good sealing performance even under severe conditions such as high temperature, or to a cell using such a gasket.
b) Description of the Related Art
Known materials of a cell gasket include polyethylene, polypropylene, copolymers of polyethylene and polypropylene, polyolefine thermoplastic resin, various kinds of rubber and the like, as described in JP-A-SHOU-53-84122 and JP-A-SHOU-55-50573. Of these materials, polypropylene in particular is easy to be molded and inexpensive so that gaskets made of polypropylene are widely used for any shape of cells including a coin type, a cylinder type and the like, and or any type of cells including alkali cells such as nickel hydrogen cells and non-aqueous electrolyte cells such as lithium cells.
However, after a long term use of alkali electrolyte cells and non-aqueous electrolyte cells as secondary cells, an inner gas pressure of these cells may rise or the cells are exposed to a high temperature so that a creep phenomenon or cracks of the gasket material are easy to occur. Many problems are therefore likely to occur, such as leakage of electrolyte, lowered cell performance caused by reduced electrolyte or by mixed water contents. In order to solve these problems, other materials have been proposed. For example, polyamide resin containing 20 to 30 weight % of alkali resistant engineering plastics (polyethylene terephthalate, polybutylene terephthalate, or the like) as described in JP-A-HEI-5-211057, polypropylene having a specific Rockwell hardness as described in JP-A-SHOU-60-35452, and polypropylene having a specific elastic modulus of flexure as described in JP-A-SHOU-61-58157. It cannot be said that these materials are effective in practical use, and improvements thereof have been desired.